The invention relates to the exchange of data between communication devices in a point-to-multipoint configuration. It has particular relevance to packet-switched communication systems.
In point-to-multipoint communication systems wherein a central node transmits and receives data to and from multiple terminals, the central node must utilise a mechanism to determine which of the terminals is allowed access to the transmission medium at any one time. This mechanism is commonly termed the media access control (MAC) protocol. In packet or cell switched networks, such as ATM networks, the MAC typically divides the transmission bandwidth into time slots. These slots are then allocated to terminals for transmitting data. Since the central node controls access to the transmission medium, it sends information or transmission permits to the terminals specifying in which future time slot data may be sent. Permits may be sent as part of payload data sent to the terminals from the node. Alternatively, the central node may send one or more permits separately, for example at scheduled times. In most cases, transmission permits are allocated only to those terminals that have traffic awaiting transmission. Information about the amount of data requiring transmission is sent to the central node. This information may be tagged onto payload data transmitted to the central node in response to a permit allocating a time slot. Alternatively, the information may be sent as a part of a separate packet in response to a polling request from the central node.
In any access control routine it is important that data packets sent in the two directions do not interfere with one another. Conventional systems allow simultaneous transmission and reception by allocating different frequencies to data in the two directions. However, simply separating the frequencies does not safely prevent interference. Specifically, there is still a potential for the transmitter to disturb the receiver in the same node or terminal when these are active simultaneously. This interference requires a frequency filter to be built into the terminal. Efficient use of the transmission capacity requires the central node to transmit and receive continuously and simultaneously. However such filters are extremely costly and when used in each remote terminal represent a substantial portion of the total production costs of such terminals.
There is thus a need for a control mechanism that reliably prevents interference between the transmitter and receiver circuitry in a terminal, but at a substantially reduced cost compared to prior art arrangements.
A method is proposed for controlling data communication between a central node and a plurality of terminals connected to the central node via a transmission medium. The central node transmits frames of data downstream to said terminals in timeslots and said terminals transmit data packets upstream in timeslots allocated by transmission permits contained in the downstream data. When transmitting a transmission permit to a terminal the central node reserves a time period in the future downstream data stream during which data destined for that terminal will be blocked from transmission. The reserved time period commences at a time location in the future data stream and has a duration, such that data transmitted in a timeslot immediately following the reserved time period will arrive at the terminal after the same has terminated transmission of data in response to the permit.
By scheduling transmission such that permits for some terminals are blocked for a predetermined and finite period of time, the simultaneous transmission and reception at one and the same node can be reliably prevented.
The time period corresponds at least equal to the time required by the terminal for transmitting a data packet. Preferably there is a delay between commencing the reserved time period and transmitting the permit, with this delay corresponding to the processing delay within the terminal. The processing delay is the time required for a terminal to commence outputting a data packet after receipt of a permit. If the time required to terminate transmission of the data in the downstream frame containing the permit is greater than the processing delay, the remaining data must either be always destined for another terminal, or alternatively, a priority must be established between the data packet in the frame and the permit. When the permit, or rather or return data packed from the terminal, has a higher priority, the permit will be transmitted and the data packet will be delayed. Data that is either destined for another terminal or contains dummy information will then be transmitted in the downstream timeslot. This data will not be affected by the blocked time period. If, on the other hand, the downstream data packet has a higher priority than the upstream data packet, the permit for the same terminal will be delayed and a permit for another terminal, or a dummy permit, will be transmitted.
When permits are issued on a regular or scheduled basis, such that their transmission timeslot or the interval between transmission timeslots is known in advance, the central node preferably also blocks a further time period immediately before the timeslot containing a scheduled permit. In this way it can be ensured that a scheduled permit will always arrive at the destination terminal after this terminal has finished transmitting data in response to an earlier permit. The duration of this second time period preferably corresponds to the terminal processing time and the time required for transmitting a data packet. In other words, transmission to the terminal is blocked for a period which covers the time required by a terminal to receive, process and respond in fill to a permit from the central node.
The invention further resides in an arrangement in the central node that includes means for selecting transmission permits and data packets for transmission in each timeslot to said terminals. Means are further provided for storing a schedule of the downstream transmission timeslots, wherein for each terminal, the schedule is blocked for a time period after transmission of a permit to said terminal. The time period terminates at an interval after transmission of the permit that corresponding to the delay required in the terminal for receiving the permit and outputting a data packet in response to the permit. The selecting means are adapted to consult the storage means and select transmission permits and data packets for terminals that are not blocked for a current time slot.